The present invention relates to an electric power steering apparatus for a motor vehicle which assists a steering wheel force by using the driving torque of an electric motor.
As a conventional electric power steering apparatus of this type, there has been known one provided with a ball thread mechanism which converts the driving torque of the electric motor to the thrust force of a rack shaft for meshing with the pinion of a steering shaft (see, e.g., Japanese Unexamined Patent Publication No. HEI 9-136655). In the conventional electric power steering apparatus, the rack portion for meshing with the pinion of the steering shaft is constructed to have a variable gear ratio which changes in response to a steering wheel rotation angle. Specifically, a rack portion has a gear ratio such that, when the steering wheel rotation angle is small, a variation in steered wheel angle responsive to the variation in steering wheel rotation angle is small and, when the steering wheel rotation angle is large, a variation in steered wheel angle responsive to the variation in steering wheel rotation angle is large. Such a rack portion having a variable gear ratio has the advantage of improved operability in each of a region where a vehicle drives straight ahead and a region where the vehicle corners since the response of the vehicle is slow in the straight-ahead region with a small steering wheel rotation angle, while it is quick in the cornering region with a large steering wheel rotation angle. Since the large steering wheel rotation angle reduces the gear ratio, the steering force of a steering wheel is reduced. This offers the advantage of reducing a required auxiliary steering force and providing a driving torque required for the generation of the auxiliary steering force even if the electric motor is small in size.
However, if the rack portion of a steering shaft has a variable gear ratio as in the conventional electric power steering apparatus for a motor vehicle, the gear ratio changes as the driver turns up the steering wheel and a variation in steered wheel angle responsive to a variation in steering wheel rotation angle changes accordingly, which causes an abnormal feeling in the driver.
If the rack portion of the steering shaft has a variable gear ratio and the lock to lock of the steering wheel is not increased compared with the case where the rack portion has a constant gear ratio, a desired overall steering gear ratio is not achieved.
The present invention has been achieved in view of the foregoing circumstances. It is therefore an object of the present invention to prevent the driver from feeling abnormal during a steering operation and reduce the lock to lock of a steering wheel in an electric power steering apparatus for a motor vehicle which assists a steering wheel operation performed by using an electric motor.
To attain the object, the present invention has provided a rack shaft with first and second rack portions, of which the first rack portion for meshing with the pinion of a steering shaft has a constant gear ratio and the second rack portion for meshing with the pinion of an electric motor has a variable gear ratio.
Specifically, the present invention aims at an electric power steering apparatus for a motor vehicle which assists a steering wheel operation by using an electric motor, the apparatus comprising: a rack shaft; first and second rack portions provided in the rack shaft; a pinion of a steering shaft for meshing with the first rack portion; and a pinion of the electric motor for meshing with the second rack portion.
The present invention specifies that the first rack portion is constructed to have a constant gear ratio and the second rack portion is constructed to have a variable gear ratio.
Since the first rack portion for meshing with the pinion of the steering shaft is constructed to have a constant gear ratio, a desired overall steering gear ratio is achievable even if the lock to lock of a steering wheel is reduced compared with the case where the first rack portion is constructed to have a variable ratio. In short, the lock to lock of the steering wheel can be reduced to a small value. Even if the steering wheel is turned up, a variation in steered wheel angle responsive to a variation in steering wheel rotation angle is constant since the first rack portion has the constant gear ratio. This allows the driver to perform a steering operation with a steady steering sense and prevents the driver from feeling abnormal.
In the electric power steering apparatus, the second rack portion is preferably constructed to have a gear ratio such that a driving torque of the electric motor is nearly constant irrespective of a steering wheel rotation angle.
If the second rack portion is constructed to have a gear ratio such that the driving torque of the electric motor changes in response to the steering wheel rotation angle, the driving torque of the electric motor changes as the driver turns up the steering wheel. Since a steering counterforce changes with the changing driving torque of the electric motor, the driver feels abnormal. If the second rack portion is constructed to have a gear ratio such that the driving torque of the electric motor is nearly constant irrespective of the steering wheel rotation angle, by contrast, the steering counterforce does not change even when the steering wheel is turned up. This prevents the driver from feeling abnormal.
Preferably, the second rack portion is constructed such that the variation in steered wheel angle responsive to the variation in steering wheel rotation angle is smaller with a large steering wheel rotation angle than with a small steering wheel rotation angle.
If the steering wheel rotation angle is large, the steering force of the steering wheel is large. Accordingly, the auxiliary steering force exerted by the electric motor should also be increased. If the second rack portion is constructed such that the variation in steered wheel angle responsive to the variation in steering wheel rotation angle is smaller with the large steering wheel rotation angle than with the small steering wheel rotation angle, the gear ratio between the pinion of the electric motor and the second rack portion provides a large auxiliary steering force even if the driving torque of the electric motor is small. This allows the use of a small-sized electric motor.
If the second rack portion is constructed such that the variation in steered wheel angle responsive to the variation in steering wheel rotation angle is smaller with the large steering wheel rotation angle, the response of the electric motor becomes slower. However, the steering wheel rotation angle is large when the vehicle is driving at a low speed, e.g., when the vehicle is being garaged. Therefore, the slower response of the electric motor does not cause a problem. In the case of counter steering, the steering wheel rotation angle is large even when the vehicle is driving at a high speed. However, since a tire is skidding during counter steering, the counterforce of the tire is extremely small. Accordingly, the required auxiliary steering force is small and the slow response of the electric motor does not cause a problem.
Preferably, the second rack portion is constructed such that the variation in steered wheel angle responsive to the variation in steering wheel rotation angle is nearly constant irrespective of the steering wheel rotation angle when the steering wheel rotation angle is smaller than a specified angle and that the variation in steered wheel angle responsive to the variation in steering wheel rotation angle decreases with an increase in steering wheel rotation angle when the steering wheel rotation angle is equal to or larger than the specified angle.
In the straight-ahead region where the steering wheel rotation angle is smaller than the specified angle, the second rack portion is constructed to have a nearly constant gear ratio so that the auxiliary steering force responsive to a steering wheel operation has a nearly constant characteristic. As a consequence, the steering counterforce responsive to the steering wheel operation is nearly constant in the region where the vehicle drives straight ahead, which prevents the driver from feeling abnormal.
At this time, the second rack portion is preferably constructed to have a gear ratio such that a driving torque of the electric motor is nearly constant irrespective of the steering wheel rotation angle. The arrangement prevents the steering counterforce from changing even when the steering wheel is turned up and thereby prevents the driver from feeling abnormal.